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Deferred income tax - Composition of the provision for income taxes (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of Deferred income tax [Line Items]      
Current $ (9,924) $ (11,911) $ (16,882)
Deferred (15,506) 37,501 (9,997)
Total income tax (25,430) 25,590 (26,879)
Income Tax Expense [Member]      
Disclosure of Deferred income tax [Line Items]      
Current (8,446) (11,851) (12,433)
Deferred (15,270) 37,617 (10,029)
Total income tax (23,716) 25,766 (22,462)
Mining Royalties and Special Mining Tax [Member]      
Disclosure of Deferred income tax [Line Items]      
Current (1,478) (60) (4,449)
Deferred (236) (116) 32
Total income tax $ (1,714) $ (176) $ (4,417)